The present invention relates generally to roadway safety devices and more particularly, to a guardrail beam having enhanced stability which minimizes failure and provides for more reliable, predictable response of the guardrail beam during a vehicle collision or impact.
A goal of roadway safety is to provide a forgiving roadway and adjacent roadside for errant motorists. Guardrail systems are employed along a roadside to accomplish multiple tasks associated with roadway safety. Upon vehicle impact, a guardrail must react as a brake and shock absorber to dissipate kinetic energy of the vehicle. Subsequently, the guardrail acts as a mechanical guide to redirect the vehicle away from hazards during deceleration and to prevent the vehicle from leaving the roadway, becoming airborne or rebounding into lanes of moving traffic. For many years, standard heavy gauge metal guardrail, often referred to as the xe2x80x9cW-beamxe2x80x9d, has been used on the nation""s roadways to accomplish these tasks and others. Named after its characteristic shape, a W-beam guardrail system is typically anchored to the ground using posts made of metal, wood or a combination of both.
Recently, there has been a vigorous effort to raise performance standards which guardrails must satisfy. Increasingly stringent testing criteria have uncovered some deficiencies in performance of standard W-beam guardrails. Considering the pervasive extent of use of standard W-beam guardrails, performance deficiencies, that have come to light as more comprehensive and rigorous test specifications are implemented, must be addressed.
A typical highway guardrail system is formed from a plurality of standard heavy gauge metal W-beams or panels which are overlapped with respect to each other at a standard splice. Depending upon the location of impact and kinetic energy associated with an impacting vehicle, interlocking or overlapping standard W-beams or panels may rotate relative to each other at the splice.
Upon a vehicle""s impact with a guardrail, a dynamic response is obtained from the guardrail. The response may include vibration of the guardrail in a direction parallel to the ground and perpendicular to the direction of the vehicle. A standard W-beam guardrail may respond somewhat effectively when the waves are in a direction away from the vehicle. However, as the standard W-beam guardrail returns in a direction toward the vehicle, standard W-beams tend to buckle or crimp at the top and bottom edges. At this point, the standard W-beam""s ability to absorb energy by plastic moment is significantly deteriorated. Furthermore, as the vehicle continues its path along the guardrail, it interacts with the edge of any buckled W-beam sections. This may result in tearing of the associated W-beam material starting at the top edge or bottom edge and may often occur in the region where two standard W-beams are overlapped.
Accordingly, recent efforts have focused on the development of a new guardrail system that will accomplish safety goals more effectively. One such design included a deeper and wider xe2x80x9cW-beam.xe2x80x9d However, this change in geometry generally requires a significant increase in hardware to attach adjacent beams or panels at each splice. Such alternative systems have not gained widespread industry acceptance because they have typically lacked the ability to efficiently interface with existing guardrail systems.
Other efforts have attempted to address typical failure modes associated with standard W-beams through system changes without changing the guardrail panel design itself. As a result, these efforts have met with only limited success, since the combination of crash event variables which may produce typical failure modes are both numerous and diverse. Thus, simply changing the configuration of a guardrail system or installation in this way generally offers little promise of significant improvement during vehicle impact.
The present invention achieves significant guardrail system performance enhancements by improving the guardrail panel design itself. Simple, precise design changes in specifically combined ratios according to the present teachings provide the basis for these novel and unexpectedly synergistic enhancements to stability and strength during service. The significance of the present invention is amplified by the ability of the new improved designs to be retrofitable, and thus to be used for both the repair and replacement of existing installations.
One aspect of the present invention is to provide an improved guardrail system for use in various locations such as median strips and adjacent to roadways. The improved guardrail system is preferably formed from a plurality of guardrail beams having a first edge region and a second edge region formed in accordance with teachings of the present invention. Guardrail beams or panels having such edge regions are generally more fracture resistant and tend to more evenly spread stresses sustained during impact between a vehicle and the associated guardrail system. Guardrail beams or panels incorporating teachings of the present invention may thus withstand significant forces of vehicle impact while maintaining adequate safety for vehicles, passengers, and bystanders.
Another aspect of the present invention is providing cost-effective, retrofitable guardrail beams or panels which may be employed interchangeably along with, or in lieu of existing guardrail systems. Still another aspect is to provide a guardrail system capable of dissipating impact energy of a vehicle collision more effectively than existing guardrail systems.
A guardrail beam or panel incorporating teachings of the present invention typically has an elongated, rectangular configuration defined in part by a first edge region and a second edge region with a front face and a rear face disposed therebetween. At least two folds are formed in the guardrail beam and extend outwardly from the rear face to provide a typical W-beam cross-section between the first edge region and the second edge region.
The first edge region is defined in part by a first flange or edge flange and a second flange or slot flange extending generally parallel with and adjacent to the first flange. The second edge region is defined in part by a corresponding edge flange and slot flange. The edge regions cooperate with each other to create a more uniform, stable and predictable response during vehicle impact. A plurality of post bolt holes and splice bolt slots are preferably provided to allow the guardrail beam to be used interchangeably with existing guardrail systems.
Technical advantages of a guardrail beam having edge regions formed in accordance with teachings of the present invention include better stabilization against crack growth that may originate near a bolt hole, a splice bolt slot or at other locations along the length of the guardrail beam. This enhanced fracture resistance is enabled by the combined effect of radius and angle between two adjacent flanges, and is still further enhanced in a compounding manner by the appropriate choice of radius to thickness ratio. This ratio serves to accomplish the dual role, first of maximizing the amount of strain hardening in the radius region which itself serves as a barrier to crack growth, and second, of emphasizing the stiffening and constraining role of one flange with respect to the other which serves as an additional crack barrier. This dual stabilization has a compounding effect against the growth of cracks that may originate near bolt holes, splice bolt slots, and other locations along the length of the guardrail beam or panel. It results in a stronger guardrail beam which is better able to resist damage resulting from impact by a vehicle. The combined effect is so significant that for some applications the strength of resulting splice bolt slots may be increased by as much as a factor of about three (3).
Another technical advantage of a guardrail system formed in accordance with teachings of the present invention includes increased stability against crimping of associated guardrail beams or panels as compared with standard heavy gauge metal W-beam guardrails. Guardrail beams having edge regions defined in part by a first edge flange and an adjacent second, slot edge flange formed in accordance with teachings of the present invention are generally more resistant to edge crimping instabilities during a vehicle impact. This is due to the fact that for edge flanges arranged relative to one another with specific combinations of radius to thickness ratios and specific ranges of angles, the edge region is significantly stabilized and thus more resistant to crimping. For some applications, the angle preferably has a value in the range of approximately twenty-five degrees (25xc2x0) to one hundred twenty-five degrees (125xc2x0).
Another aspect of the present invention includes providing guardrail panels with at least one edge region having a first, edge flange and an adjacent second, slot flange disposed at a selected angle relative to each other and with a selected radius formed therebetween to provide greater resistance to rotation of one panel relative to another panel at a standard splice. Greater resistance to rotation during a crash event is enabled as the edge flanges more effectively interlock the panels at the splice during installation. At the same time the radius to thickness ratio ensures significant work hardening of the material in the region of the radius when work hardenable materials are used. This work hardening effect is synergistic with other aspects of the design in that it significantly strengthens the support of the cantilevered edge flange in the region of highest local bending stresses that would work to let the edge flange simply deform and thus permit rotation of one panel relative to the other at the splice. This higher strength is added without compromising the ease of installing the panels relative to each other.
It may be noted that local heat treatment, welding operations, or mechanical working of the material in the radius region may be performed to further accomplish the above strengthening effect. It may also be noted that in the case of panels that are substantially made of a sheet base metal such as steel, the thickness that is used in calculating the radius to thickness ratios is typically that of the uncoated base metal from which the panel is formed. It may also be noted that for some applications, the angles formed between the first, edge flange and adjacent second, slot flange may be varied along the length of each panel in order to achieve various design objectives for the overall system while substantially maintaining the benefits of the teachings of the present invention. The angles associated with the respective edge regions may also be varied to minimize manufacturing costs.
A further technical advantage of a guardrail panel incorporating teachings of the present invention includes a first edge region and a second edge region having more stability to resist crimping than is commonly associated with the blade edge of standard W-beam panels. Crimping may be defined as an edge buckling instability commonly associated with open section structures subjected to bending loads. Guardrail beams or panels having edge regions formed in accordance with teachings of the present invention generally resist crimping more effectively, thus substantially adding to the stability of the guardrail beam section. This in turn enables the panels to demonstrate more uniform stresses over the cross-section of the guardrail panel or beam during a vehicle impact. This more uniform stress distribution is crucial in achieving more uniform and stable system response and thus increased performance effectiveness while resisting and guiding the impacting vehicle.
Still another technical advantage includes a splice bolt slot configuration that facilitates retrofit and/or replacement of existing guardrail systems with one or more beams or panels formed in accordance with teachings of the present invention without requiring substantial modifications to existing equipment and other portions of the existing guardrail system.
The teachings of the present invention are primarily discussed within this document as being specifically applied to critical edge regions of a guardrail panel. However, the same principles and teachings may also be combined with other features or material processing steps, including local heat, pressure, laser or electromagnetic treatments, as they are applied to any region of a panel cross section of a new or existing guardrail panel design, wherever local changes in the section shape may already occur or are desired. In such cases extended local application of the present principles in a panel cross-section can be used for example to accomplish redistribution of material mass or strength. This implementation may be varied along the panel length in order to address specific design objectives. Naturally, as a panel is tailored in this way, economies of processing simplicity and material usage remain as practical considerations.
In summary, providing guardrail panels with edge regions formed in accordance with teachings of the present invention results in significantly more stable interaction between an associated guardrail system and an impacting vehicle. This is because the guardrail system is better able to resist local rotation and stresses at each splice, and the fact that the edges are significantly stabilized against crimping that is associated with instability and weakening of the guardrail section. The present invention thus enables tailored configurations of guardrail panels and their associated guardrail system in order to optimize trade offs between performance and ease of installation at each point along the length of the guardrail system. While many modifications to a standard W-beam guardrail system are possible, the embodiments of the present invention can greatly enhance the resistance of the resulting guardrail system to failure. In fact, the magnitude of this effect is both surprising and novel. Moreover, significant benefits may be provided without substantially compromising other desirable characteristics of the standard W-beam such as overall simplicity and diversity of application. The result is a new guardrail system that may be substantially more consistent, predictable, and reliable in light of current performance and testing standards.
Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.